The circumstances in which the most powerful country in the world and its closest ally would use nuclear weapons is clearly of vital importance - not only to the potential targets, but also to the people whose leaders would take the decision. Now, just at the time when the use of such weapons appears more, not less, likely, both the Bush administration and the Labour government seem determined to stifle all debate.

The Pentagon has just removed from its website a document outlining a new doctrine for joint nuclear operations for the US chiefs of staff. For the first time it sets out specific guidance for US commanders reflecting the Bush administration's doctrine of pre-emptive strikes. It envisages the use of nuclear weapons to pre-empt a possible attack by a country, terrorist or criminal group with "weapons of mass destruction".

It states: "To maximise deterrence of WMD use, it is essential US forces prepare to use nuclear weapons effectively and that US forces are determined to employ nuclear weapons if necessary to prevent or retaliate against WMD use."

The document lists examples of when nuclear weapons could be used. The first is against an enemy using or "intending to use" WMD against the US or its allies. American military commanders could ask the president to use nuclear weapons when there is the threat of an "imminent attack from adversary biological weapons that only effects from nuclear weapons can safely destroy", to attack "deep, hardened bunkers containing chemical or biological weapons", or to attack the command-and-control "infrastructure" an enemy would use to attack the US or its allies with WMD.

The proposed new doctrine significantly lowers the threshold for triggering the use of nuclear weapons, notably America's 480 tactical nuclear bombs in Europe, including the 110 at the US base at Lakenheath in East Anglia.

According to Hans Kristensen, a consultant to the US Natural Resources Defence Council, who first noticed the document on the Pentagon website, the doctrine envisages the use of US nuclear weapons in conflicts where they would previously have been considered illegal. "Instead of drastically reducing the role of nuclear weapons, as the Bush administration told the public it would do," he says, "the proliferation of weapons of mass destruction and terrorism seem to have spooked the administration into continuing and deepening a commitment to some of the most troubling aspects of the nuclear-war-fighting mentality that symbolised the cold war."

Dominick Jenkins, a Greenpeace disarmament campaigner who has also analysed the Pentagon document, observes that "where US nuclear policy leads, the UK generally follows". It is crucial, he says, that MPs and the public seriously examine them.

Unsurprisingly, Donald Rumsfeld, the US defence secretary, and General Richard Myers, chairman of the US joint chiefs of staff, play down the significance of the document. At a Pentagon press conference on September 20, Myers denied the doctrine lowered the nuclear threshold, though neither he nor Rumsfeld had yet read it, they said.

The Senate armed services committee is not fooled and has asked the Pentagon for a briefing on the new doctrine. In Britain the document has barely been noticed, and the Ministry of Defence is refusing to release any information on the government's plans to maintain a nuclear deterrent and replace the existing Trident missile system. Last week it dismissed requests for MoD documents under the Freedom of Information Act, refusing to say what studies have been made about the costs involved. It refuses even to say what nuclear weapons are for, arguing that it is not in the public interest to publish its assessments about what threats such weapons could deter.

The MoD was asked to release studies it has made assessing the threats that might be deterred by a Trident replacement. It replied that though there was a "strong public interest" in the UK having a "credible nuclear deterrent", "it is felt that releasing information about the potential value of a deterrent capability ... could damage national security, and we do not believe there would be any public interest in doing so".

The ministry also refuses to disclose the nature of discussions with the US on nuclear-weapons policy on the grounds that "there is a public interest in the UK maintaining strong relations with the US". That would be prejudiced, the MoD argues, if any information about talks with American officials was released.

In an interview with the Guardian last month, John Reid, the defence secretary, promised an open debate on any decision to replace Trident. There should be a debate in the country as well as in parliament, he suggested. In light of the blanket refusal to release any papers relating to the matter, a defence official told the Guardian: "There is no need for a debate now. When the time comes there will be a debate." That, presumably, will be when it is too late to make any difference to what the government has already decided, in private with Washington.